EVAPORATION. 



165 



liquid approaches the temperature of the vapor suspended in the air, the ten- 

 sions will approach more nearly to equality, and the resistance of the vapor 

 already suspended in the air will speedily begin to produce a sensible effect 

 on the rate of evaporation. In order, therefore, to detect the law by which 

 evaporation took place at lower temperatures, it became necessary first to de- 

 termine the actual tension of the aqueous vapor suspended in the atmosphere 

 at the time of the experiment. The properties of vapor previously discovered 

 by Dalton, led him to an elegant and simple solution of this problem. The 

 aqueous vapor suspended in the atmosphere, not being in a state of saturation, 

 must be regarded as having received a quantity of heat which dilated it and 

 raised its temperature, according to the laws for the dilatation of the permanent 

 gases after it had passed from the liquid to the vaporous state. Now if all the 

 heat which has been imparted to it after it had passed into the vaporous state 

 be taken from it, it will undergo a diminution of temperature, but will not pass 

 from the vaporous to the liquid form. The smallest abstraction of heat beyond 

 this point will, however, cause a deposition of moisture, and a partial condensa- 

 tion of the vapor. If, therefore, a body at a temperature considerably lower 

 than that of the atmosphere be exposed to the air, it will first by abstract- 

 ing heat from the vapor in contact with it, lower its temperature until it 

 arrives at that temperature which it had when it passed from the liquid 

 to the vaporous state. If the body be at a lower temperature, then, though it 

 can no longer lower the temperature of the vapor, it will condense it, and the 

 vapor will deposite itself in the form of dew on the sides of the body. If the 

 body be actually or nearly at that temperature at which the vapor passed from 

 the .liquid to the aeriform state, then the commencement of the condensation 

 will be just indicated by a slight dulness produced on the surface of the body 

 by the condensation of the smallest possible quantity of vapor. Led by such 

 reasoning, Dalton adopted the following means of determining the temperature 

 at which the vapor suspended in the atmosphere had passed from the liquid to 

 the aeriform state : He poured water, at a temperature below that of the atmo- 

 sphere, into a thin glass tumbler, and exposed it to the air. If he observed an 

 immediate and rapid deposition of dew upon its surface, he then wiped the 

 vessel dry, and exposed it at a somewhat higher temperature. He thus con- 

 tinued to expose the vessel at increasing temperatures, until he found that tem- 

 perature at which a deposition of moisture would just take place on its surface, 

 and such that one degree higher in temperature would prevent such a con- 

 densation of vapor. This, then, he assumed to be the temperature at which 

 the vapor suspended in the atmosphere had passed from the liquid to the 

 aeriform state, and the elasticity or tension corresponding to this temperature 

 was found from the table of elasticity resulting from his former experiments. 

 Now the vapor actually suspended in the air had a higher temperature than 

 this, and was raised to that temperature by heat communicated to it after it had 

 assumed the vaporous form. The additional tension imparted by this increase 

 of temperature was easily computed by the rules for the dilatation of gases and 

 vapors by heat. Hence he computed the actual tension of the vapor suspended 

 in the atmosphere. 



The water used by Dalton in this experiment was taken from deep wells at 

 Manchester, the temperature of which was from 10° to 15° colder than the 

 atmosphere. This served the purpose when the temperature of the air was not 

 very low, but in winter, when the temperature was near the freezing point, it 

 became necessary to cool the water by means of ice, or a mixture of snow and 

 salt, or other freezing mixtures. 



The deposition of condensed vapor with the appearance of dew, on the exter- 

 nal surface of a glass vessel containing iced water, is a fact of familiar occur- 



